Power supplies such as embedded power supplies and adapter power supplies for TV, PC, and server applications typically include a front-stage power factor correction (PFC) circuit and a second-stage LLC converter. PFC circuits counteract the distortion caused by non-linear devices such as rectifiers, and raise the power factor. LLC converters have a topology that utilizes a combination of two inductors and one capacitor (“LLC”) on the primary side of the converter. A switching (full or half) bridge on the primary side generates a square waveform that excites the LLC tank circuit, which in response outputs a resonant sinusoidal current that is scaled and rectified by a transformer and rectifier circuit of the LLC converter. An output capacitor filters the rectified ac current and outputs a DC voltage.
Both the PFC and LLC stages are often controlled by a single digital power controller. In the case of TV adapter power supplies, the power supply must meet standby power consumption, output voltage ripple and dynamic response requirements during standby mode. For example, some TV adapter power supplies must have an input power consumption less than 270 mW at an output load of 152 mW, an output voltage ripple less than 190 mV (<1% of 19V output voltage), and an output voltage drop less than 0.95V (within ±5% of 19V output voltage) during load jumps from 0% to 100% load.
The characteristics of a conventional LLC converter make it difficult to meet standby power consumption, output voltage ripple and dynamic response requirements for power supplies such as TV, PC and server adapter power supplies. If the LLC converter is operating at a fixed switching frequency (LLC burst frequency) in low power standby mode that is selected based on certain input voltage and standby load conditions, the output voltage ripple at other input voltage and standby load conditions may not meet the requirements. Different LLC gains and/or overly long burst-on time leads to excessive delivery of energy. In addition, the secondary-side control may saturate, leading to slower dynamic response to load changes, larger output voltage drop and higher standby power losses in the secondary side control circuitry. These problems worsen when resonant component tolerances are considered.